ARTIFICIAL MANURE MANUFACTURE 63 



where one or more constituents could be omitted 

 or reduced in quantity. It was then suggested 

 that soil analysis should form the basis of deter- 

 mining the needs of the soil for different crops, but 

 this failed to produce satisfactory results. The 

 formulas at present used by many have been based, 

 in part, upon the composition of the plant, and, 

 in part, upon actual field tests. 



The amount of nitrogen called for by analysis 

 of plants is generally reduced because we can de- 

 pend upon the soil to furnish a considerable 

 amount. In case of leguminous crops, the amount 

 of nitrogen which we need to supply can be reduced 

 to a small fraction of what the plant will use, 

 because such crops can draw their main supply of 

 nitrogen from the air. 



The amount of soluble phosphoric acid is or- 

 dinarily increased above what plant analysis calls 

 for, because the solubility is more or less de- 

 creased after the fertilizer comes in contact with 

 the soil. 



The proportion of sulphuric acid used in mak- 

 ing a superphosphate is generally as great as pos- 

 sible without impairing the dryness of the finished 

 manure. It is usually more than sufficient for the 

 realization of the equation given above. It is 

 now supposed that the reaction which occurs takes 

 place in two stages, the sulphuric acid first liberat- 

 ing an equivalent of phosphoric acid, which then 

 reacts with the remaining CasffO^. The pres- 

 ence of calcium sulphate tends to solidify the mass 



